Environmental contamination from a variety of sources is a serious and continuing problem, especially in developing countries where access to expensive remediation technologies is generally limited. Arsenic, lead, and other heavy metals are common contaminants which are desirable to remove from drinking water. Much research effort has been directed toward developing in vivo chelators to treat cases of acute metal poisoning; however, these chelators do not allow metals such as Pb(II) or As(III) to adopt their preferred coordination geometries, which may contribute to their lack of selectivity.
Self-assembly of discrete supramolecular aggregates utilizing reversible coordinate bonding interactions has provided access to elaborate and precisely designed structures that have generated an enormous amount of interest. Transition metals have been most widely used in designing coordination-driven discrete structures. In contrast, much less work focuses on using main-group elements as directing units for the construction of supramolecular structures. Recent work has demonstrated that As(III) in combination with appropriately designed bridging thiolate ligands(L) can self-assemble into a discrete dinuclear As2L3 structure, supported by a secondary supramolecular interaction between the lone pair of As(III) and the aromatic ring of L. These secondary bonding interactions (SBIs) can take many forms, as shown in the assembly of a naphthalene-imide ligand with As(III) where weak arsenic-oxygen interactions support a highly symmetric complex in solution. These SBIs may similarly work to confer selectivity for a target analyte.
Previous work has shown that AsIII and other heavier main group elements with stereochemically active lone pairs can form close contacts with arene rings in the solid state. To analyze this attractive interaction Density Functional Theory calculations were performed on the AsCl3-benzene dimer. These studies revealed a lower limit for the binding energy of 7.4 kcalmol−1 for the arsenic-arene interaction, with C˜As distances of 3.2-3.4 A°. Furthermore, although the preferred geometry of the interaction orients the arsenic lone pair at a 68° angle to the phenyl ring; this structure is only 0.5 kcalmol−1 more stable than the two C3-symmetric arrangements. This suggests that, while this interaction is quite strong, its geometry is flexible. This lone pair-π interaction may work in the constraints of this Invention to induce either stronger binding or the possibility of selectivity for a given target.
Raymond in U.S. Pat. No. 5,049,280 (1), included herein by reference, teaches the use of a ligand to bind iron in solution. A portion of the ligand may comprise an organic linking group, A, bonded to a solid substrate, Q. The compound chelates or sequesters one metal ion per compound. These compounds are termed herein “monofunctional” in that one molecular ligand attaches or sequesters one target specie, in Raymond's case a metal ion such as Fe+3. In J. Chem. Soc., Dalton Trans., 1999, 1185 (2), included herein by reference, Caulder and Raymond discuss various supramolecular structures. The simplest multi-metal cluster contains two metal sites linked by one or more ligands. When these two metal ions are linked by three identical C2-symmetric ligand strands, written as M2L3, the resulting bimetallic cluster is called a triple helicate if both metal ions have the same chirality; an example is dihydroxamine siderophore rhodotorulicacid; other examples are found in the reference. Catecholamide and hydroxamate ligands are excellent choices for binding units in supramolecular complexes because of the high stability and lability of these chelates with +3 metal ions with octahedral coordination environments; Hydroxypyridinone and pyrazolone ligands are also useful in synthesizing supramolecular clusters. M4L6 complexes are also discussed wherein the vertices of a tetrahedron act as interaction or coordination sites for four metal ion species and six ligands act as edges of the tetrahedron. These ligand compounds are termed herein “bifunctional” for the two specie case and “multifunctional” for the higher order cases.
Turner, in “Molecular Containers: Design Approaches and Applications”; Structure and Bonding, Vol. 108, 2004, 97 (3); included herein by reference, discusses numerous “molecular containers”, completely enclosed hollow species capable of holding one or more guest species inside, and their ability to form a covalent assembly of “guest-encapsulating host species”; the first examples of host species binding their guests within a three-dimensional array of interactions were the class of compounds known as cryptands, discussed in a 1969 article; typically cryptands are synthesized by the addition of a diacyl-chloride to an azacrownether. Turner describes other molecular shapes such as a cavitand, which can be likened to a deep bowl in which a guest can reside, shown in FIG. 6A. Turner describes carcerands and hemicarcerands wherein a guest specie is enclosed by a cage compound; one way to envision this structure is two cavitands linked such that the bowl openings of each cavitand are facing each other, similar to a clam shell with a guest specie trapped inside.
Castellano, in “Formation of Discrete, Functional Assemblies and Informational Polymers through the Hydrogen-Bonding Preferences of Calixarene Aryl and Sulfonyl Tetraureas”; J. Am. Chem. Soc. 1998, 120, 3657 (4); included herein by reference, discusses calixarenes with urea functions and how they create capsules. Castellano describes a dumbbell arrangement of two capsules and a 3 capsule arrangement in a triangular shape. All four previous references demonstrate the ability of a ligand of varying complexity to bind, covalent or labile, a “guest” specie; none of the references teach the ability of a bi- or multi-functional ligand to bind to a series of “guests” as shown in FIGS. 3 and 6B.
A metal complex, also known as coordination compound, is a structure composed of a central metal atom or ion, generally a cation, surrounded by a number of negatively charged ions or neutral molecules possessing lone pairs. Counter ions often surround the metal complex ion, causing the compound to have no net charge. The ions or molecules surrounding the metal are called ligands. Ligands are generally bound to a metal ion by a coordinate covalent bond, and are thus said to be coordinated with the ion. The process of binding to the metal ion with more than one coordination site per ligand is called chelation. Compounds that bind avidly to form complexes are thus called chelating agents (for example, EDTA). Coordination numbers, or the number of bonds formed between the metal ions and ligands, may vary from 2 to 8. The number of bonds depends on the size, charge, and electron configuration of the metal ion. Some metal ions may have more than one coordination number. Different ligand structural arrangements result from the coordination number. A coordination number of 2 corresponds with a linear geometry; a coordination number of 4 corresponds with either a tetrahedral or square planar molecular geometry; and a coordination number of 6 corresponds with an octahedral geometry. Simple ligands like water or chlorine form only one link with the central atom and are said to be monodentate. More examples of monodentate ligands include hydroxide, nitrite, and thiocyanate. Some ligands are capable of forming multiple links to the same metal atom, and are described as bidentate, tridentate etc. Oxalate and ethylenediamine (en) are examples of bidentate ligands, while diethylenetriamine (dien) is a tridentate ligand. EDTA is hexadentate, which accounts for the great stability of many of its complexes. Herein the terms coordination site, attraction site, binding site, linking site and interaction site are used approximately equivalently.
Previous work with coated or surface-modified zeolites may be found in U.S. Pat. No. 6,080,319 (5) and U.S. 2004/0108274 (6); both disclose methods for adsorbing contaminants, including pathogens, onto a porous substrate such as zeolite. U.S. Pat. No. 6,838,005 (7) teaches a nano-porous, synthetic substrate of aluminum hydroxide fibers for adsorption purposes. None of the examples teach a method to increase the adsorptive capacity of a substrate or filter by adding additional layers or coatings of an adsorption enabling material such as a bi- or multi-functional ligand.
Conventional adsorbents, including carbon, zeolites and synthetic resins from Dow Chemical or Rohm & Haas, have limited capacity for contaminants; a typical loading capacity ranges from below 0.01% by weight to maybe as high as 0.5% by weight, depending on the conditions. In general a contaminated adsorbent is then disposed of in a hazardous landfill, an expensive situation. Heavy metals such as lead, mercury and other pollutants such as arsenic and uranium are expensive to remove from water and expensive to dispose of. With the scarcity of potable drinking water the problem is acute. There is a need for an inexpensive adsorbent for contaminants in water which has increased capacity and ease of disposal. Other applications and embodiments of the instant invention are included herein as will be apparent to one knowledgeable in the art.